This invention relates to a system for scanning a document, which is positioned on a flat document support. The invention has particular application to systems which scan a flat document surface and generate a series of electrical signals which are suitable for controlling an ink jet printer or other electrically operated dot matrix recorder. One such prior art system is disclosed in Paranjpe et al U.S. Pat. No. 4,112,469.
In the scanning system disclosed in Paranjpe et al there is a collimating lens, which collimates light reflected from the document surface and an imaging lens for imaging a scanned portion of the document upon the face of a photodetector array. A scanning mirror is positioned between the collimating lens and imaging lens. This system has a problem in that the arcuate scanning of the flat document plane produces variations in the length of the optical path from the scanned document portion to the face of the photodetector array. This tends to defocus the observed image during portions of the scan, and to produce variations in the relative sizes of the scanned portion of the document and the detector array, which results in a geometric distortion of the dot matrix image.
One prior art system which avoids defocussing problems arising out of arcuate scanning of a flat document plane is disclosed in Herriott U.S. Pat. No. 2,262,584. Herriott teaches a scanning apparatus for facsimile recording, wherein light reflected from a portion of a flat mounted document is collimated by a moving doublet lens. The doublet lens moves in synchronism with the scanning action of the scanning mirror, so that the focal plane of the doublet lens always coincides with the portion of the document being scanned. Herriott provides another doublet lens, which is supported in a fixed position relative to a photoelectric cell. The photoelectric cell is positioned at the focal plane at the second doublet lens. The photocell views a series of side-by-side image elements arranged along a line extending across the width of the document. The document is moved on a line by line basis, as each line of image cells is scanned.
Other systems for achieving flat field scanning includes reciprocating mirrors as disclosed in Satomi U.S. Pat. No. 3,709,602 and Lloyd U.S. Pat. No. 3,752,558 and sidewardly moving lenses as disclosed in Nishiyama et al U.S. Pat. No. 3,681,527. Still other prior art systems have utilized combinations of mirrors and lenses as shown for example Yuta U.S. Pat. No. 4,025,154.